Moscow is threatening to restart the testing of nuclear weapons, and warns of a possible atomic war, if the United States proceeds with the development and testing of tactical or "bunker buster" nuclear weapons.

"If Washington takes this step, Russia will be free to review its stand on nuclear tests," Moscow declared, according to the Voice of Russia World Service, the official broadcasting service of the Russian government.

Not only will Russia begin testing, but also there also will be a "mass exodus by a whole group of nations" from the nuclear test ban treaty, Moscow asserted. U.S. testing of tactical nuclear weapons will lead to "many other countries" developing their own atomic arsenals, and the "threshold of a possible nuclear war," Moscow warned.

"The United States will lose all moral right to pose as the champion of non-proliferation," protested Moscow.

Top U.S. officials have been discussing the possibility of developing small, tactical nuclear weapons, especially following the 9-11 attacks.

The Robust Nuclear Earth Penetrator, if developed, would be designed to crash through concrete and steel and then detonate.

The concept originated in the Cold War, when the U.S. sought to penetrate well-constructed, deep underground Soviet bunkers.

Development of a "bunker buster" weapon gained renewed attention as nations suspected of aiding terrorist groups adopted the same bunker strategy as employed by the defunct USSR, but with improved Western technology.

For over a decade, nations associated with terror groups have spent millions of dollars on reinforced steel and concrete, multi-level bunkers. The bunkers protect troops, weaponry, and even factories, according to reports.

Libya and North Korea, close allies of Moscow, have the most extensive bunker systems, and are consistently linked to terrorist activities.

North Korea shares nuclear and rocket technology with Iran, as it continues to periodically threaten neighboring South Korea and Japan with destruction, and assists Libya's long-range missile program.